Most tanks (and aspiring tanks) want to be the main tank of a raid. It is upon this prestigious player's shoulders that the success or failure of the raid is often considered to rest.
First of all, we know that stereotype is untrue. Healers and DPS both play critical roles in raid success, even though they are sometimes forced to argue for their significance. But I don't often hear praise for the off tanks. While the healers heal, the DPS damages and the main tank tanks, these "wannabe main tanks" play critical roles in most every fight. Whether they have to pick up adds or share the burden of boss tanking with the main tank, these intrepid heroes have to work just as hard to keep the raid alive.
I first tasted off-tanking back in Naxxramas (10-man) when I stepped up to the task of being one of the rear tanks on my elemental shaman during the Four Horsemen fight. Here, I received my first taste of the special joy of adapting my skill set to new situations for every fight. I pined to be the one who would kite the zombies on Gluth, but my shaman lacked the capability, and one of our mages shouldered the responsibility. You can rest assured, however, that I was always back there to assist by laying down my Stoneclaw Totem.
My guild fell apart for a while after Ulduar was released, and I was too poor for more WoW. Once I wasn't poor anymore, I leveled a feral druid, and we all got back together just recently and started raiding Icecrown. Now I'm the off tank, and I'm loving it. Being an off tank lets me flex and show how creative and adaptable I am.
Do any of you tanking types out there prefer the off-tank role?